There is a growing demand for high performance, high stability secondary batteries as electric, electronic, communication, and computer industries have rapidly developed. Particularly, in line with miniaturization and lightweight trends of electronic (communication) devices, thin-film and miniaturized lithium secondary batteries, as core components in this field, are required.
Lithium secondary batteries may be divided into a lithium ion battery using a liquid electrolyte and a lithium polymer battery using a polymer electrolyte depending on the electrolyte used.
With respect to the lithium ion battery, it may have high capacity, but, since the liquid electrolyte containing a lithium salt is used, there may be a risk of leakage and explosion and battery design may be complicated to prepare for the risk.
With respect to the lithium polymer battery, since a solid polymer electrolyte or a gel polymer electrolyte containing an electrolyte solution is used as the polymer electrolyte, stability is improved and, simultaneously, flexibility is obtained, and thus, the lithium polymer battery may be developed in various forms, for example, in the form of small or thin-film batteries. In particular, in a case in which the gel polymer electrolyte is used, since the number of components used in the preparation of the lithium secondary battery may be reduced, a cost-saving effect may also be expected.
However, since the polymer electrolyte has significantly lower ionic conductivity than the liquid electrolyte, the polymer electrolyte may not be suitable for commercialization.
For example, with respect to polyethylene oxide which has been widely used as the polymer electrolyte, it has an excellent ability to dissociate an ion conductive metal salt despite the fact that it is in a solid state. That is, since cations of the alkali metal salt are stabilized while the cations are coordinated with oxygen atoms present in the polyethylene oxide to form a complex, the cations may be present in a stable ionic state without a solvent. However, since the polyethylene oxide has a semi-crystalline structure at room temperature to interfere with the movement of the metal salt in which a crystal structure is dissociated, it is disadvantageous in that energy characteristics are degraded, for example, it has a low ionic conductivity value of about 1.0×10−8 S/cm at room temperature. Thus, it may not be suitable for commercialization.
Recently, a hybrid polymer electrolyte or gel polymer electrolyte, in which an ionic conductivity of 1.0×10−4 S/cm or more is obtained by adding several to nearly ten times as much as the amount of a liquid electrolyte solution to a polymer matrix, has been studied.
Typical examples of the gel polymer electrolyte may be a copolymer (Panasonic Corp) obtained by mutually copolymerizing heterogeneous monomers selected from the group consisting of polyacrylonitrile (EIC Lab. Inc.) vinyl chloride, vinyl acetate, acrylonitrile, styrene, and methyl acrylate monomer; a copolymer (Nippon Telegraph & Telephone Corporation) of a high polar monomer, such as vinyl chloride, methyl methacrylate, vinyl alcohol, and acrylic acid, and a low polar monomer such as styrene and butadiene; a polymethyl methacrylate-based copolymer having high affinity with the electrolyte solution, and a terpolymer (Samsung General Chemicals). However, with respect to a conventional polymer electrolyte, it is difficult to prepare the polymer electrolyte having both excellent mechanical properties and lithium ion conductivity.
Thus, there is an urgent need to develop a polymer electrolyte material having high ionic conductivity, processability, and mechanical properties while maintaining a solid phase.